This invention refers to a system or plant for molding polyurethane articles, comprising molds supported by carriages or trolleys movable along a suspended monorail.
In the mass production of polyurethane articles, there are substantially two types of molding systems, in which the molds are made to move along a pre-established path and through various work stations in order to carry out the operations of opening the molds to remove the molded articles, cleaning and filling the molds with metered quantity of a foamable polyurethane mixture, and closing the molds to permit the final stage of polymerization of the resin.
According to the first type of plant, a limited number of molds are secured to a rotary table. The table is made to turn step by step with pre-established intervals of time, in order to bring each individual mold in correspondence with the various work stations. This type of system or plant has the advantage of allowing the polyurethane mixture to be fed in with the mold in a stationary condition. However, it presents numerous drawbacks such as, for example: the limited number of molds that can be used, difficulty and long pauses for the removal, replacement and/or maintenance of the individual molds, with a consequent reduction in the productivity of the system, and lastly, no or very little operative flexibility which limits the possibility of adapting or utilizing a system for various production requirements. Moreover, the rotary table systems are complicated in structure, take up a considerable amount of space and do not offer the operator easy access to the open molds. In order to at least partially remedy the limited productive capacity and the lack of operative flexibility of rotary table systems, so-called "roundabout" systems have been designed, in which the individual molds are mounted on their respective carriages which move on wheels along a circular path, on one side of which are arranged the various work stations; the individual carriages are interconnected by means of a motor driven chains.
According to this type of plant, the hydraulic control devices are connected to a steady source of fluid under pressure by means of flexible pipes and a rotary distributor, or to movable sources of fluid under pressure which also run on carriages along the production line. Compared to the previous rotary table system, this system offers the advantage to arrange the production line differently and to remove each individual carriage from the work path by conveying them towards a separate area for maintenance and/or replacement of the mold. However, the operation of disconnecting and reconnecting a carriage to the driving chains and to the source of fluid under pressure is complicated and difficult to carry out when the carriage is moving. Consequently, it is necessary to stop the entire production line for the length of time required for removing and/or replacing one carriage with another. Moreover, since it is virtually impossible to stop the entire line of carriages in positions perfectly aligned with the apparatus for injecting or feeding the polyurethane into the individual molds, the polyurethane material must be fed with the carriage in motion, along a short section of its working stroke. Consequently, whereas on the one hand the movable carriage system offers the advantage of a different disposition and organization of the plant, and the possibility of removing and replacing the carriages, stopping the plant just long enough to disconnect and reconnect them to the driving chains and to the feed conduits for the fluid under pressure, on the other hand it calls for the plastic material to be fed with the molds in motion, which proves to be somewhat undesirable and somewhat unadvisable for molds of large dimensions or with particular shapes, in that it is difficult to obtain a correct and even distribution of the polyurethane in the mold when operating with the molds in motion.
Moreover, the same carriage system cannot easily be adapted to meet the various production requirements, due to the fact that the carriages are all made to move simultaneously at the same speed which, like the rotary table system, must make allowances for the working times established by the entire production process.
It would therefore be desirable to have a polyurethane molding system which combines the advantageous features of the previous systems, such as the possibility of feeding the polyurethane mixture with the mold stationary, in a suitable position, as in the rotary table systems, and the possibility of supporting the molds so that they can be easily replaced, in a separate area from the working line, as in the present carriage type systems, thereby permitting greater flexibility in terms of control, operation and design and ridding the system of any dangerous obstacles on the ground, along the path of the carriages and in particular in the areas in which an operator intervenes.
Consequently, an object of this invention is to provide a polyurethane molding plant capable of achieving the above-mentioned advantages while at the same time greatly simplifying the structure, operation and control of the system itself by providing an independent and programmable drive mechanism for each individual mold.
A further object of this invention is to provide a plant of the aforementioned type, which offers the possibility of positioning the individual molds on carriages which can be made to move independently from one another, even at different speeds, along the entire path of the production line, and to stop from time to time at the various work stations without affecting the movement of the other molds.
A still further scope of this invention is to provide a plant as described, in which the individual mold supporting carriages can be guided and diverted from the main production line towards one or more separate service areas, with the possibility of returning to the production line whenever required, without hindering or interfering with the movement of the remaining molds. A still further scope so this invention is to provide a plant with mold supporting carriages completely independent from one another, which offers the greatest freedom in designing and locating the various work stations and the possibility of creating additional or alternative paths, and which can all be monitored automatically by a central logic unit.